This invention relates to a Raman laser and in particular but not exclusively a Raman laser for producing multiple rotational Raman orders.
There are only certain materials or combinations of materials convenient for use in a laser, and consequently only a limited number of wavelength bands which can conveniently be obtained by lasing action and each of these is usually narrow. In certain applications it is desirable to use other frequencies, to be able to switch between different frequencies, or to broaden the frequency spectrum of the band.
In 1928 Sir Chandrasekhara Vankata Raman observed an effect now known as Raman scattering. This occurs when energy in the form of photons incident on a molecular structure raises the energy state of a molecule to an intermediate, or virtual state, from which it makes a Stokes transition emitting a photon of energy, termed a scattered photon. The scattered photon may have the same energy as the incident photon or alternatively a higher or lower energy. To have a higher or lower energy the energy value of the molecule must have changed. The molecule can obtain or release this energy in the form of vibrational or rotational energy. Because distinct vibrational and rotational energy levels exist scattered photons also have distinct energy values and therefore the incident beam of photons is in effect frequency shifted by "scattering". This effect is most commonly achieved using molecular gases such as H.sub.2, D.sub.2, CH.sub.4 or CO.sub.2, which are commonly referred to as being Raman active.
Raman scattered light consists of vibrationally scattered and rotationally scattered components which are side bands of the incident laser frequency. The molecule has a larger separation between vibrational energy states than rotational energy states and thus "vibrational shifts" in frequency are greater than "rotational shifts" in frequency. One of the prime uses of the Raman effect has so far been in analytical chemistry whereby the change in frequency gives an indication of the energy level structure of a molecule.
The most efficient conversion of light using the Raman effect requires that a laser beam incident on the molecular gas is above the threshold intensity for stimulated Raman scattering (SRS). When moderate powered lasers are employed Raman scattering can be enhanced by focusing the pump beam in the gas by means of lenses or mirrors. However the use of focusing optics makes it difficult to maintain boresight stability as movement of these optics will deflect the beam.